Switching power supplies are used to drive many types of loads. At times, it is desired to drive multiple loads using a single switching power supply requiring only a single inductor. This is particularly desired when driving multiple light emitting diode (LED) loads as it saves board space and money. Instead of using a power supply module to drive each string of LEDs, a single power supply module may be used to drive multiple LED strings. But as with many load types, it may be important to properly regulate the power delivered to the load.
A commonly used switching power supply topology uses current mode control. Current mode control, as usually implemented in switching power supplies, actually senses and controls peak inductor current. This may give rise to many serious problems, including poor noise immunity, a need for slope compensation, and peak-to-average current errors which an inherently low current loop gain cannot correct. Average current mode control eliminates these problems and may be used effectively to control currents other than inductor current, allowing a much broader range of topological application.
Current mode control is a two-loop system in which the switching power supply inductor is located within the inner current control loop. This simplifies the design of the outer voltage control loop and improves power supply performance in many ways, including better dynamics. The objective of this inner loop is to control the state-space averaged inductor current, but in practice the instantaneous peak inductor current is the basis for control. In many designs, switch current, which is equal to inductor current during the “on” time of the switch, is often sensed.
In a conventional switching power supply employing a buck derived topology, the inductor is located in the output. Current mode control then is actually controlling the output current, resulting in many performance advantages. On the other hand, in a boost topology, the inductor is located at the input. Current mode control then controls input current. The technique of average current mode control introduces a high gain integrating current error amplifier (CA) into the current loop. A voltage across a sense resistor represents the desired current program level. The voltage across the current sense resistor represents actual inductor current. The difference, or current error, is amplified and compared to a large amplitude sawtooth (oscillator ramp) at the PWM comparator inputs. The gain-bandwidth characteristic of the current loop can be tailored for optimum performance by the compensation network around the CA. The average current mode method can be used to sense and control the current in any circuit branch. Thus it can control output current with boost topologies, for example.
LEDs are semiconductors with light-emitting junctions designed to use low-voltage, constant current DC power to produce light. LEDs may use an average current, boost mode switching power supply. LEDs have polarity and, therefore, current only flows in one direction. Driving LEDs is relatively simple and, unlike fluorescent or discharge lamps, they do not require an ignition voltage to start. However, too little current and voltage will result in little or no light, and too much current and voltage can damage the light-emitting junction of the LED diode.
When lighting designers arrange a series of LED strings in applications such as street lights or industrial lights, each string have been driven at a consistent current by an individual LED driver. However, the output voltage often varies due to differences in the manufacturing of the LEDs. To compensate, LED drivers may be configured to provide higher-than-needed voltage to ensure proper operation of each LED string. Too much voltage, though, can waste power.
With a typical LED forward voltage vs. forward current profile, for a given temperature, a small change in forward voltage produces a disproportionately large change in forward current. In addition, the forward voltage required to achieve a desired light output can vary with LED die size, LED die material, LED die lot variations, and temperature.
As LEDs heat up, the forward voltage drops and the current passing through the LED increases. The increased current generates additional heating of the junction. If nothing limits the current, the junction may fail due to the heat. This phenomenon is referred to as thermal runaway.
Light output of LED light sources increases with increasing drive current. However the efficiency, expressed in lumens per watt, is adversely affected. Drive currents may be chosen at any current up to the maximum recommended current for the specific LED light source used. Driving LED light sources above the maximum recommended currents may result in lower lumen maintenance or, with excessive currents, catastrophic failure.
In non-dimming applications, a constant-current driver is chosen to deliver the desired current, with enough forward voltage output to accommodate the maximum input voltage of the LED source. LED light sources are not designed to be driven with a reverse voltage.
By driving LED light sources with a regulated constant-current power supply the light output variation and lifetime issues resulting from voltage variation and voltage changes can be significantly reduced. Therefore, constant current drivers are generally recommended for powering LED light sources.
Conventional AC-DC power supplies and DC-DC converters provide an output that is regulated to provide a “constant-voltage.” However, LEDs work most efficiently and safest with a “constant-current” drive. LED power sources that provide a “constant-current” output have typically been referred to as LED drivers. However, there are heretofore unaddressed needs with previous solutions for driving multiple LED outputs with a single inductor in the switching power supply power train, which include crosstalk and inefficiency.